graphical peak discharge method

8/10/2019 Graphical Peak Discharge Method

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Graphical Peak Discharge Method

The EquationIn the Graphical Peak Discharge Method, runoff is calculated using the following formula:

qp= quAmQ Fp

Where:

qp= peak discharge (ft3/s, also written as cfs)

qu= unit peak discharge (cfs/mi.2/in., also written as csm/in)

Am= drainage area (mi.2)

Q = runoff (in.)

Fp= pond and swamp adjustment factor (no units)

Procedure

The steps used to calculate runoff using the Graphical Peak Discharge Method are as follows:

Step 1- Measure the drainage area (Am).

To translate the drainage area from acres to square miles, use the following equation:

Drainage area (acres) 0.00156 = Drainage area (mi.2)

You will also need to know the drainage area in square feet in order to calculate total runoff

volume. To translate drainage area from acres to square feet, use the following equation:

Drainage area (acres) 43,560 = Drainage area (ft.2)

Step 2 - Calculate a curve number (CN) for the drainage area.

In order to calculate the CN, you will need to know the soil type(s) and hydrologic soilgroup of each soil type. You also need to know the land use and vegetative cover. Then

you can determine the CN usingTable 5.

Curve numbers for heterogeneous watersheds may be determined by dividing the watershed into

homogeneous sub-areas and performing a weighted average as in the rational method.
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Step 3- Determine runoff depth and volume for the design storm.

First, the rainfall depth (P) is found usingDiagram 5.Then the rainfall depth and the CN

are used withDiagram 6to find the runoff depth. Then the total runoff volume can becalculated using one of the following two equations:

Step 4- Determine time of concentration (Tc).

Step 5- Determine initial abstraction (Ia).

Initial abstraction is found using the CN andTable 7.

Step 6 - Determine the unit peak discharge (qu).

First calculate the Ia/P ratio using Iafrom Step 5 and P from Step 3.

Next, determine the unit peak discharge using the Ia/P ratio and the time of concentration

withDiagram 8.

Step 7- Determine the pond and swamp adjustment factor ( Fp).

First measure or estimate the area in the watershed covered by ponds and swamps.

Next, calculate the percent ponds and swamps using the following equation:

Finally, find the pond and swamp adjustment factor usingTable 8.
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Step 8 - Determine the peak discharge (qp).

Example

Given InformationThe following watershed is found in western Lee County:

For present conditions, the flow path was determined to be 4700 feet long by using field surveys

and topographic maps. Reach AB is 200 feet of sheet flow in woods and light brush at 2% slope.

Reach BC is 500 feet of shallow concentrated flow over an unpaved surface at 4% slope.

Reach CD is 1500 feet in a natural channel with 8 square feet cross sectional area, 7.6 feet wettedperimeter, 2% slope and a Manning's "n" of 0.08.

Reach DE is 2500 feet in a natural channel with 27 square feet cross sectional area 21.6 feetwetted perimeter, 0.5% slope and a Manning's "n" of 0.06.

The watershed drainage area is 157 acres. Of this total drainage area, 39 acres is woods and lightbrush in fair condition, 79 acres is urban open space in fair condition, and 39 acres is a

residential district with an average lot size of 1 acre. The residential area overlies Tumbling loam

and the open space and woods overly Berks soil.

1.5 acres of the watershed is ponds and 0.5 acres is swamps.

GoalUse the Graphical Peak Discharge Method to calculate the peak discharge and total runoffvolume for the present, pre-developed watershed.


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Solution1. Drainage Area

1. In square miles:

157 acres 0.00156 = 0.245 mi.2

2. In square feet:

157 43,560 = 6,838,920 ft.2

2. Curve Number

1. Divide the drainage area into three regions:

Region 1:39 acres of woods and light brush in fair condition onBerks soil (Hydrologic Soil Group C). CN = 73

Region 2:79 acres of urban open space in fair condition on BerksSoil (Hydrologic Soil Group C). CN = 79

Region 3:39 acres of residential district with 1 acre lots on Tumbling

Loam (Hydrologic Soil Group B). CN = 68

2. Calculate the CN:

3. Runoff Depth and Volume

1. Rainfall depth = 3.0 inches. (From Diagram 5.)

2. Runoff depth = 0.95 inches. (From Diagram 14.)

3. Volume of runoff =

4. Time of Concentration


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1. Sheet flow travel time:

1. n = 0.40. (From Table 3.)

2. Travel time:

2. Shallow concentrated flow:1. Velocity (from Diagram 1) = 3.2 ft./sec.

2. Travel time:

3. Channel flow:

1. Reach CD:

a. Calculate "r".

b. Velocity:

c. Travel time:


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2. Reach DE:a. Calculate "r".

b. Velocity:

c. Travel time:

4. Total time of concentration:

Tc= 0.643 + 0.043 + 0.153 + 0.341

Tc= 1.18

5. Initial Abstraction= 0.667 inches. (From Table 7.)

6. Unit Peak Discharge

1. Ia/P ratio:

2. qu= 300 csm/in. (From Diagram 8.)


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7. Pond and Swamp Adjustment Factor

1. Percentage ponds and swamps:

2. Fp= 0.85. (From Table 8.)

8. Peak Discharge

qp= (300) (0.245) (0.95) (0.85)

qp= 59.35 cfs

Results

The total volume of runoff from the watershed is 541,415 cubic feet.

The peak discharge for the watershed is 59.35 cfs.

graphical peak discharge method

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